1. Field of the Invention
The present invention generally relates to the art of microelectronics, and more specifically to a resistor ladder which can be electrically programmed or trimmed even after the circuit in which the resistor ladder is incorporated has been encapsulated in a package.
2. Description of the Related Art
Resistors can be advantageously formed on microelectronic integrated circuit substrates by screen printing using special resistive inks which dry to form thick or thin films. The process is economical in that all of the resistors can be formed on a substrate simultaneously, and the screen printing process is inherently inexpensive.
A drawback of this process is that it is not especially precise. Deviations of 20% for thick film resistors and 10% for thin film resistors from their design resistances is not uncommon.
For precision applications, film resistors are trimmed by selective removal of material using a laser or other device to achieve the required resistance. A description of laser trimming of film resistors is presented in a textbook entitled "HYBRID MICROCIRCUIT TECHNOLOGY HANDBOOK", by J. Licari, Noyes Publications, Park Ridge, N.J., 1988, pp. 132-148.
Laser trimming is an expensive and time consuming process, requiring each resistor to be trimmed individually. Another drawback of laser trimming is that it cannot be performed after a microelectronic circuit has been encapsulated in a protective package.
A typical microelectronic circuit will include many film resistors having different resistances. In the conventional design process, each resistor must be individually configured in accordance with its required resistance. The layout of the circuit is therefore complicated, time consuming and expensive.